Automatic precipitator control



y 1958 R. G. STREUBER v 2,843,215

AUTOMATIC PRECIPITATOR conmor.

Filed July 2. 1953 5 Sheets-Sheet 1 9 INVENTOR.

RUDOLF s. STREUBER.

ATTORNEYS 15, 1958 R. G. STREUBER AUTOMATIC PRECIPITATOR CONTROL 5 Sheets-Sheet 2 Filed July 2, 1953 SATURATIQ WINDING SATURATING WINDING.

INVENTOR. RUDOLF G. STREUBER.

K n/14f 5M ATTORNEYS.

y 15, 1958 Q R. G. STREUBER 2,843,215

AUTOMATIC PRECIPITATOR CONTROL Filed ,Iul 2, 1953 I 5 Sheets-Sheet a SATURATING wmoms.

SATURATING WINDING.

INVENTOR.

RUDOLF G.STREUBER By my WM ATTORNEYS y 1953 R. s. STREUBER 2,843,215

AUTOMATIC PRECIPITATOR CONTROL Filed July 2, 1953 5 Sheets-Sheet 4 INVENTOR. RUDOLF G. STREUBER ATTORNEYS 62 i so 31/1/ July 15, 1958 R. a. STREUBER AUTOMATIC PRECIPITATOR CONTROL 5 Sheets-Sheet 5 Filed July 2. 1953 mmvron RUDOLF G. STREUBE R ATTORNEYS United States Patent AUTOMATIC PRECIPITATOR CONTROL Rudolf G. Streuber, Somerville, N. 1., assignor to Research Corporation, New York, N. Y., a corporation of New York Application July 2, 1953, Serial No. 365,602

14 Claims. (Cl. 1837) This invention relates to electrostatic precipitators for gas cleaning, and more particularly to high voltage directcurrent supplies utilizing automatic control for such precipitators.

For the most efficient operation, electrostatic precipitators require as high a voltage between the discharge and collecting electrodes of the precipitator as it is practical to maintain. An upper limit of voltage is imposed on the one hand by the danger of a heavy are or flash-over which composes substantially a short circuit upon the system and on the other hand by the average frequency of routine sparking, a certain optimum rate of which has been found to improve the etficiency of operation. In the case of a full short circuit across the line it is customary to employ normal overload protective devices, and the invention is not intended to alter or affect this type of operation.

A primary object of the invention is to operate the high voltage circuit of a precipitator as close to the highest value of voltage and current as any given precipitator condition may require for best results, permitting an optimum amount of sparking or slight arcing where operation makes this desirable, but applying correction instantly in the event of a power are, restoring optimum operating conditions immediately upon cessation of the are or any tendency to have it sustained. This object is accomplished by a modification of the high voltage transformer, which is conventional in all precipitator systems, by providing a saturating winding which so controls the magnetic circuit of the high voltage transformer as to produce the desired results.

Another object is to provide a unitary high voltage transformer with a saturating winding so arranged as to eliminate substantially all ditliculties due to wave distortion tending to result from interlinkage of the power and control fields, and to provide stable control behavior under all normal conditions of operation.

Still another object is to provide a control system for an electrostatic precipitator, including a saturable control winding for the high voltage transformer thereof, for both maintaining slight sparking in the precipitator at an optimum rate and also applying instantaneous correction in the event of a moderate power are with immediate restoration'of normal operation under such conditions and without operating the normal heavy overload protective devices, and thus interfering with operation.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from the following description of a preferred embodiment'as shown in the accompanying drawings, in which:

Fig. 1 is a schematic drawing of a transformer with a saturatingwinding according to the invention;

Fig. 2 is a view similar to Fig. 1 but showing a modified construction capable of accomplishing the same result;

Fig. 3 is a schematic view of the core of the transformer of Fig. 1, showing the flux distribution with the primary and saturating winding both energized at below saturation;

Fig. 4 is a view similar to Fig. 3 but showing the flux distribution with the saturating winding fully energized;

Fig. 5 is a circuit diagram showing the transformer of Fig. 1 employedin a basic control circuit for a precipitator;

Fig. 6 is a similar control circuit showing an electronic tube in the saturating winding;

Fig. 7 is a schematic diagram of a control circuit with a saturating winding employing a full wave tube rectifier as a source of anode current;

Fig. 8 shows a similar control circuit for the saturating winding employing two thyratrons essentially as recti- Fig. 9 is a schematic circuit diagram of a detailed arc-over and spark control circuit; and

Fig. 10 shows a modification of Fig. 9 in which the saturating winding is energized through two triodes connected for full wave rectification.

Referring to Fig. 1, the transformer employed has a modified shell type core 2. Leg 4 conforms to standard design principles. Legs 6 and 8 combined have the crosssectional area of leg 4, one-half in each leg. Leg 10 is equal in cross-sectional area to 4, but has a small air gap 12. Primary 14 and secondary winding 16, which are the main transformer windings, are placed on leg 4 in the customary manner. The control or saturating winding 18 is placed on legs 6 and 8, one half-winding on each in such manner that opposite magnetic polarity is induced in these legs. By means of the end sections 7 and 9 between legs 6 and 8, a closed magnetic circuit is provided with few if any lines of force travelling outside of this path. The sections 7 and 9 connecting the ends of the legs are equal in cross-section to 4.

Fig. 2 shows an arrangement very similar to Fig. 1 except that the main winding leg 4a and the leg 10a having a gap in it are reversed in position. The operation will obviously be the same as for Fig. 1.

Fig. 1 shows the magnetic flux with only the primary winding energized. Since alternating current is used, the flux direction of course varies with the current, and the arrows therefore represent the flux at a particular instant. It will be seen that nearly all of the flux in leg 4 divides equally between legs 6 and 8. There is some flux in leg 10 but owing to the high reluctance of this path due to the air gap, it is negligible. The alternating flux in legs 6 and 8 produces a voltage in each half of saturating coil winding 18. These voltages, however, oppose one another and so neutralize each other, so that the net effect and coupling are zero. The coupling between primary 14 and secondary 16 is normal maximum, according to design, and the transformer action is normal.

Fig. 3 illustrates the flux with current in both the primary winding and the direct current control winding well below saturation of its magnetic circuit. It will be understood that the control winding is energized with direct current, and it will be noted that practically none of the flux produced by this current will travel outside of the closed magnetic circuit formed by legs 6 and 8 and interconnecting end sections 7 and 9. Even if alternating current were applied to the saturating winding 18, coupling to the primary and secondary would be substantially zero because of the above described flux path. There is therefore no inductive effect from either magnetic field on the coil or coils of the other field, so that wave distortion or erratic interactions are avoided. This is an important feature of the invention. Fig. 3 shows that with the particular instantaneous polarity indicated for the primary winding, the flux direction due to the primary in leg 6 is opposed by the flux produced by the saturating winding 18. (With reverse polarity for the primary winding, such as will occur a half cycle later, the same would be true for leg 8 instead of leg 6.) As is well known, due to the flux opposition in leg 6, and to the now increased saturation in leg 8 of the control circuit, much of the flux caused by the current in primary winding 14 is now forced into the high reluctance leg 10 with consequent loosening of the coupling betweenprimary-and secondary due to the weakermagnetic field, roughly in proportion to the energization of saturating winding 18.

In Fig. 4, complete saturation of legs 6 andS is assumed with the result that the flux due to current in the primary 14 is now confined to the high reluctance leg 10 with still further reduction in magnetic field strength. This'represents the full control condition, and the minimum available output from the secondary winding.

It should be noted that in designing the parameters of the above transformer for use in a precipitator control circuit according to the invention, such values will be selected as not to reduce the coupling between primary and secondary for the condition of Fig. 4 below a value at which precipitator voltage would be subjected to too drastic a'reduction. The reduction should not be greater than barely to suppress the power arc or to guard against a sustained arc. Guarding against heavy grounds or shorts in the precipitator should remain the function of the over-load devices provided for this purpose. It should be noted that the saturable winding effect of the proposed control is the reverse of heretofore proposed arrangements. In the arrangements referred to, maximum saturation usually results in maximum values in the controlled circuit, whereas in the proposed control, maximum saturation results in minimum transformer values. At zero saturating current the transformer performs strictly in accordance with standard design principles.

Fig. S'showsthe transformer of Fig. l employed in a basic control circuit. The output of high voltage secondary winding is supplied to rectifier 22 shown schematically as'a full-wave rectifier. It will be understood that at the voltages normally employed in precipitation work, this rectifier would either be a conventional mechanical rectifier, or else employ vacuum tubes as the rectification elements but may also be a selenium rectifier. The rectified high-voltage output is applied between discharge electrode 24 and collector electrode 26 of a precipitator as schematically indicated. A resistor 28 is provided in the collectorelectrode circuit and a portion of the voltage drop through this resistor is supplied to control winding 18 through variable tap 30 whereby the setting of the control current'may be varied to suit the requirements. It will be apparent that upon the occurrence of a heavy discharge between the electrodes of the precipitator, the resultant increase in current can be made to saturate the control winding and thus reduce the output of the secondary winding 16 of the transformer to a point too low to sustain the are, thus enabling resumption or normal operation.

It will be noted that in the arrangement of Fig. 5, which is intended primarily to be illustrative of the basic principle, no provision is made to block current to the saturating winding entirely for normal precipitator operation. This feature is provided in Fig. 6, by inserting an electronic tube 32 in the saturating winding circuit, the constants being such that conduction occurs only when correction is required. It will be understood that the circuit of Fig. 6 would be connected at the points x, y of Fig. 5 in place of the corresponding circuit elements shown in the latter figure.

Fig. 7 is a similar circuit arrangement showing the use of a full-wave tube rectifier 34 as a source of'a'node current, only the signal circuit being connected to the precipitator circuit dropping potentiometer 28b. This provides the advantage that the control circuit supply voltage remains at a fixed value. in Fig. 6 power is furnished by the main transformer high voltage circuit through the potentiometer. Operation of the circuit depends primarily on three factors:

1) Voltage drop between points of connection of the control circuit which, of course, will vary with the current in the precipitator circuit and which will raise the plate voltage at the time that the control should furnish correction.

(2) Relative values of potentiometer and control circuit which are in parallel and which will cause a reduction of joint resistance when the tube is conducting.

(3) Setting of the grid control point. Where thyratrons are used as shown the grid will control only initiation of current flow during each pulse through the thyratron in accordance with the point in each pulse cycle at which the instantaneous grid voltage reaches the critical value to initiate conduction. If vacuum tubes are employed, closer control may be obtained, but for precipitation operation thyratrons will usually be found to be satisfactory.

Fig. 8 shows the use-of two thydratrons 36 and 38 operating in effect as grid controlled rectifier. This provides higher current capacity than the arrangement of Figs. 6 or 7 and the controlpower is again independent of the signal circuit. No reset capacitor such as shown at 29 in-Fig. 6, is required for this circuit as each tube has a half-cycle period in whichto reset. In all the above figures, grid bias is schematically represented by battery 33, although in a practical circuit this would normally be derived from a conventional voltage divider arrangement.

Fig. 9 shows a more comprehensive arrangement based upon the fundamental control circuit of Fig. 5. Transformer saturating winding 18a is connected in series with the main transformer secondary winding 16a, inserted between the high voltage rectifier 22a and the grounding resistor 31, but may also be connected between the lower end of the grounding resistor and ground.

Across the terminals of the saturating winding 18a amotor operated potentiometer 40 is connected which, by virtue of its position, provides a range of control from full saturation current to practical-1y zero saturating current. It has an off-position at 42 in which the potentiometer is on open circuit and the total transformer secondary current is through the saturating winding. The motor 44 is indicated as a universal motor, but may also be a reversible induction motor. It is connected to the secondary of a small control transformer 46 through the contacts of the two balancing relays-48 and 50 which in turn are part of a control bridge circuit. One side shown as the left of the bridge circuit consistsof a portion of the manual control potentiometer 52 in series with the control thermistor 54, the left balancing relay 48 and a portion of the motor operated balancing potentiometer 56, depending upon the instantaneoussetting of the latter. The right hand side consists of the remainder of the manual control potentiometer 52, of a matching balancing thermistor 58, the second balancing relay 50 and of the remainder of the motor operated balancing potentiometer 56. The two halves of the bridge are energized in parallel from the control transformer secondary which also supplies energy to the motor circuit.

The rest of the circuit consists of the spark counting portion. A capacitor. plate or similar device is placed in the proximity of the high voltage conductor 62 or cable terminal which carries current to the precipitator. Every sparkin'the precipitator causes a fluctuation in charge on the plate 60 which is in thegrid circuit of thyratron 64 whose components are arranged so that it is just below conduction from plate to cathode. This-change in chargecausesthe tube to fire-and current to exist in the'load resistor 66. The load resistor 66 is enclosed in a compartment as'indicated by the dotted lines, with thermistor'54 in such manner that the heat generated by the current in the resistor 66 is not directly radiated to the thermistor, but rather so that an average temperature is reached within a predetermined time which may be one, three, or five minutes according to the design.

The best precipitator operation is secured when the operating voltage is sufficiently high to cause moderate arcing at a uniform rate. This rate varies for different precipitator conditions, but is in general somewhere between 150 and 300 sparks per minute. In order to illustrate operation, an optimum sparking rate of 175 per minute is assumed. The precipitator is energized in the customary manner by closing the primary transformer circuit and adjusting the secondary voltage to a desired value by means of primary rheostat 68, or alternatively by using a tapped auto transformer or induction regulator, or any other well known method of control. Then the control circuit is closed at 70. Arcing in the pre cipitator causes the thyratron load resistance to heat up, reaching an average or mean temperature within the time limit of parameter design. The resistance to current flow through the thermistor S4 is lowered as its temperature increases. Thus the bridge circuit becomes unbalanced. More current exists in the left-hand leg than in the right and the left balancing relay 48 closes its contacts. This in turn energizes the motor 44 which will turn in a clock-wise direction transferring more of of the balancing potentiometer resistance from the right to the left leg until balance is reestablished and the left balancing relay 48 opens its contacts. At the same time the saturating winding control potentiometer contact 41 moves to place more resistance in parallel with the saturating winding thereby shunting more of the transformer secondary current through the saturating winding, and thus providing a stronger saturating field in the transformer core.

At start of operation, however, this automatic control feature is prevented from operating in this manner by the manipulation of the manual control potentiometer 52, the contact of which is moved by hand to reestablish the necessary bridge balance before the automatic control functions, and thus precipitator operation is adjusted initially for maximum secondary voltage as indicated by the desired sparking rate of 175 sparks per minute. From this point on no further manual control is required even though precipitator conditions may change to require either higher or lower secondary voltage for optimum operation. A change in the direction of the first condition will be reflected by a lowering of sparking rate, permitting a cooling of the load resistor in the thyratron plate circuit with consequent cooling of the thermistor in its proximity. Therefore, the thermistor resistance increases and the bridge becomes unbalanced to cause rotation of the motor in a counter-clockwise direction until bridge balance is reestablished by automatically transferring resistance of the balancing potentiometer to the right leg of the bridge. This is accompanied by reducing the resistance value across the saturating winding and thereby shunting more of the secondary transformer current around the saturating winding and so weakening the saturating field.

A change in precipitator operating conditions in the opposite direction will be reflected in an increase in sparking rate With consequent reverse effect upon the control circuit and corresponding reverse action.

It is evident that the sparking rate is the sole controlling factor and that the intensity of the arc-over in the precipitator is not a factor. By design all portions of the thyratron plate circuit are of constant value and the current value in it will be the same for each fluctuation in pick-up plate charge attended by conduction through the tube.

However, since the saturating winding is in series with the transformer secondary every current variation in the latter, such as an increase in current due to a heavy arcover must result in a similar variation of current in the saturating winding with consequent corrective action,

even though the regular control circuit lags behind and controls only average change in precipitator conditions. Thus momentary disturbances which express themselves in heavy arcing of short duration are also subject to control and prevent frequent trip-outs of the equipment which might otherwise occur.

This is a notable feature in which the proposed control differs from control schemes which have been proposed heretofore and the time lag of recuperation which is inherent in saturable reactors in the transformer primary circuit is almost completely eliminated.

Fig. 10 is a modification of Fig. 9. The control scheme is substantially identical except that in Fig. 10 the saturating winding 18b is energized through two triodes 72 connected for full-wave rectification. The control of plate current through the triodes is by automatic grid control actuated by the motor and potentiometer follower as before, through lead 74. It furthermore provides for automatic control of the resistance 31a in the grounded hightension leg by means of a further resistor 78 in series with triode 76, whose grid potential is controlled at 80 by a mechanical link 82 operated by the control motor 44a, thereby increasing or reducing the series voltage drop across the resistor and so deducting from or adding to the high tension voltage available for precipitator operation. An arbitrary example will be assumed for the purpose of explanation:

Grounding resistor 31a=20,000 ohms. In parallel with it is a 10,000 ohm resistor 78 in series with a triode 76.

This triode is chosen for maximum conduction at negative grid potential, to block at say -40 v. grid potential.

Assuming, furthermore, normal precipitator current at in 100 ma. at which the tube would conduct fully, then winding would be fully energized for maximum control.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.

I claim:

1. A system for supplying high voltage direct current to the electrodes of a high voltage electrical precipitator comprising a transformer having a core comprising two end yokes and between the end yokes a main leg carrying primary and secondary coil windings, two adjacent and mutually similar legs each carrying half of a saturating winding and a high reluctance leg which includes an air gap, whereby when no current flows through the saturating winding the magnetic circuit through the main leg is completed by said two similar legs in parallel and when a high current flows through the saturating winding the magnetic circuit through the saturating winding is completed by said high reluctance leg, means for rectifying the output of the secondary winding and applying it to the electrodes of the precipitator, and means for deriving a control voltage from said rectified output and applying it to the saturating winding whereby the output of said secondary winding is controlled by said saturating winding in response to variations in the voltage conditions at the precipitator electrodes.

2. The invention according to claim 1 wherein said control voltage deriving means comprises a resistance element in series with said precipitator electrode having tapped means for contacting said resistance element at spaced points thereon and leads between said tapped means and said saturated winding.

3. The invention according to claim 2, including a vacuum tube amplifier in at least one of said leads, said amplifier having a control grid connected to one of said taps and a platecircuit connected to said saturating winding.

4. The invention according to claim 3, the plate supply being derived from an'additional tap on said resistance element.

5. The invention according to claim 3, including a separate plate voltage supply independent of said control voltage deriving means.

6. A system for supplying high voltage direct current to the electrodes of ahigh voltage electrical precipitator comprising a variable-ratio saturating-core transformer having a low voltage primary winding, a high voltage secondary winding and a saturating winding so arranged that the degree of saturation of the saturating winding controls the effective coupling between the primary and secondary windings, means for rectifying the output of the secondary winding and applying it to the electrodes of the precipitator, means for deriving a control voltage from said rectified output and applying it to the saturating winding whereby the output of said secondary winding is controlled by said saturating winding-in response to variations in the voltage conditions at the precipitator electrodes, and means responsive to sparking between the precipitator electrodes for integrating the rate of said sparking, means responsive to said integrated rate for controlling the output of said secondary winding to maintain an optimum integrated sparking rate, whereby momentary sparking disturbances and the integrated sparking rate are respectively controlled by said saturating winding and said spark integrating control.

7. The invention according to claim 6, including pickup means responsive to sparking between the precipitator electrodes to produce an electric pulse corresponding to each spark, an integrating circuit including a resistance element for dissipating thermally the energy of said pulses, a thermally insulated enclosure for said resistance element, a temperature-sensitive electrical resistance element in said enclosure, a similar temperature-sensitive resistance element exposed to the ambient temperature, an automatic rebalancing bridge circuit for balancing the resistance of said elements to a predetermined equilibrium, comprising motor means responsive to an unbalance in said bridge circuit for rebalancing same, control potentiometer means actuated by said motor means, and connections between said saturating winding and said control potentiometer means for controlling said saturating winding in accordance with the integrated sparking rate.

8. The invention according to claim 7, including a manually controllable potentiometer in said bridge circuit for adjusting the initial setting of the sparking rate.

9. The invention according to claim 8, including a series dropping resistor in said rectified output circuit for further controlling the voltage output to said precipitator electrodes,'a variable shunt across at least part of said dropping resistor and means for varying said shuntin accordance with the position of said motor means.

10. The invention according to claim 9, said last means including a shunt resistor in series with a grid-controlled triode having variable grid bias means, and a mechanical control link between said last means and said motor means.

11. The invention according to claim 10, said connections between the saturating winding and said control potentiometer means including thermionic grid-controlled rectifying means, a separate plate supply therefor for energizing the saturating winding, and connections between said control potentiometer means and the gridcircuit of said grid-controlled rectifying means.

12. A variable-ratio saturating-core step-up transformer having a core comprising two end yokes and between the end yokes a main leg carrying low voltage primary and high voltage secondary coil windings, two adjacent and mutually similar legs each carrying half of a saturating winding and a high reluctance leg which includes an air gap, whereby when no current flows through the saturating winding the magnetic circuit through the main leg is completed by said two similar legs in parallel and when a high current flows through the saturating Winding the magnetic circuit through the main leg is completed by said high reluctance leg.

13. The invention according to claim 12 wherein the combined cross-sectional area of the two legs carrying the saturating winding is equal to that of the leg carrying the primary and secondary windings.

14. The invention according to claim 12 wherein the two halves of the saturating winding are wound on the two similar legs in opposite senses whereby a magnetic field caused by current in the saturating winding is confined substantially to said two legs and the parts of the yokes between the ends of said two legs.

References Cited in the file of this patent UNITED STATES PATENTS 1,995,652 R'eichard Mar. 26, 1935 2,029,628 Lord Feb. 4, 1936 2,245,192 Gugel June 10, 1941 2,466,028 Klemperer Apr. 5, 1949 2,519,426 Grant Aug. 22, 1950' 2,632,522 Fields Mar. 24, 1953 2,672,947 Klemperer Mar. 23, 1954 

